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Reformulating the Problem There are many ways of categorizing explanations for the apparent paradox. One is to focus on actions: what we think the extraterrestrials are doing, and what we are doing. Another is to concentrate on the intentions of each civilization; many explanations rest on assumptions about the behavior of alien societies, which, in turn, rest on analogies with our own. We could classify explanations by technological capabilities, ours and theirs; or we might do as some authors have done, mixing these approaches. Here is another way to categorize our speculations, one that tries to take into account the serious explanations proposed by others. Where possible, the approach outlined below avoids making our Earthly perspective the center of the question, looking at these issues from a more detached point of view. We assume not only the spatial context of a huge galaxy containing hundreds of billions of stars, but also a temporal context that reaches back billions of years. The time frame includes not only the present but also a very long past. This formulation addresses only technological civilizations, defined here as those capable of employing the technologies of interstellar communica- tion and/or interstellar flight or other technological activities that we can detect, such as astroengineering. There could be civilizations that are capable of using such technologies but do not do so. There may be far more that do not have these capabilities at all. (Our own civilization was one of those until recently.) Here the sentient beings that form technological civi- lizations include postbiological intelligences, the descendants or masters of biological beings. We should be cautious about applying any generalized explanation to all extraterrestrial civilizations. Technological societies sprung from different evolutions in different environments may do different things; we cannot assume uniformity in their behavior. Even if there is a collectivity such as a Galactic Club, there may be individual societies that are not members or that break its rules. Most of the suggested alien behaviors that would make contact more likely involve the commitment of resources—financial, physical, or other. 180